1. Field of the Invention
The present invention relates to a device to scrape and/or meter a liquid or viscous medium on a moving operating surface.
2. Description of the Related Art
During operation of such a device, undesired deformations of a doctoring support beam often occur relative to the operating surface. In the paper industry, the main reason for these deformations is the uneven heating of the doctoring support beam, wherein that side of the doctoring support beam facing the operating surface receives more heat from the comparatively warmer medium, such as coating ink or glue, than does the side of the doctoring support beam facing away from the operating surface. A temperature gradient occurs in the doctoring support beam which leads to a deflection of the metering rod support. A deformation occurs in which the doctoring support beam deflects in the center toward the operating surface but remains at its edges a relatively greater distance from the operating surface. The result is an uneven cross profile of the medium remaining on the operating surface. In other words, there is an uneven coating thickness of applied medium across the width of the working surface.
Particularly on relatively large operating surface widths with correspondingly wide doctoring support beams, i.e. supports of 8 to 10 meters or wider, a trough-shaped coating profile occurs if no special compensation devices are provided. This phenomenon is discussed in an article "New development in the area of coating equipment for paper and carton: Control of coating cross profile" by Dr. Ing. H. P. Sollinger--reprint from "Wochenblatt fur Papierfabrikation" No. 23/24 (1989), Pages 1072-1076. From this article, methods of compensating for the deflection of the doctoring support beam are also known. As a result of the methods, the achieved coating cross profile is less trough-shaped and more uniform. Under the method of a pneumatic contact pressure system for the doctoring component, a pneumatically activated contact pressure tubing is held on the doctoring support beam and presses the doctoring component against the operating surface. The flexible contact pressure tubing is equipped to partially compensate for unevenness, particularly such unevenness as is found in the deflection of the doctoring support beam. In practical terms, however, it has been found that such pneumatic flexible tubing provides an insufficient averaging of the coating profile. Particularly because of the material-related non-homogeneity and the comparatively rough manufacturing tolerances of the pneumatic flexible tubing, new irregularities with relatively large peak deviations are created. Also, the trough-shaped characteristics of the cross profile cannot be completely compensated for.
A method for additional compensation is described in the aforementioned article in which deformations of the doctoring support beam are intentionally caused. The deformations are intended to counteract the undesired doctoring support beam deflections.
In a thermal system, separate water circuits can be used to produce different temperatures on the front and back sides of the doctoring support beam. By suitable targeted adjustment of water circuit temperatures, a desired deformation of the doctoring support beam can be effected to the point that a straight doctoring support beam is achieved.
In a mechanical system, several flexible pressure tubes are provided in a doctoring support beam. The pressure tubes may effect the targeted deformation of the doctoring support beam by suitable pressure application. Both the thermal and the mechanical systems permit total compensation of operationally dependent, undesirable deformations of the doctoring support beam, and lead to an almost perfectly uniform coating cross profile. However, a disadvantage of this compensation system which directly affects the doctoring support beam is the high cost of the component parts of the compensation system, as well as the high cost of its control. First of all, the linear deviations of the doctoring support beam must be measured. This is usually done by laser measurement, which is very cost intensive. These laser measurement results must then be converted into control signals, which will control the temperatures in the water circuits and the pressure in the flexible pressure tubing. Complicated control algorithms are necessary for comparison purposes. Finally, the component part: expenditure for the control circuit, including the water circuits and the flexible pressure tubing, is considerable.
The current invention provides a low cost method of compensating for undesirable deformations in the doctoring support beam and of achieving a substantially uniform coating cross profile in an above-described type of doctoring device.